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Hydrozoans are an important component of both the benthos and the plankton in all oceans. Their ranges extend from polar to tropical waters, and from the intertidal zone to ocean trenches. Some penetrate into estuaries, but only a small percentage of known species occur in fresh waters. Most are free living, although a few are parasitic. Approximately 3400 species are currently recognized worldwide. Life cycles may include a polyp or hydroid stage only, a medusa stage only, or an alternation between the two. Hydroids are generally sessile, colonial, and epifaunal; hydromedusae are typically free swimming, solitary, and planktonic; siphonophores are colonial and largely planktonic . The taxonomy of hydrozoans is still in a primitive state. Nomenclature and classification of the group have been encumbered from the time of Linnaeus (1758) to the present by essentially separate systems for hydroids and hydromedusae. In many taxa having metagenetic life cycles, polypoid and medusoid stages of a given species have frequently been given different scientific names and even assigned to different classifications. Advances have been made over the past century toward eliminating the dual systems of nomenclature and classification through life-cycle studies. Stability, universality, and uniqueness of names can be promoted once a link between different life stages has been established by following provisions of the International Code of Zoological Nomenclature. Vestiges of these problems remain, particularly in taxa for which life-cycle information is lacking or in groups where morphologically dissimilar medusae have virtually indistinguishable hydroids, or vice versa. Questions also arise concerning the classification of groups in which certain taxa liberate free medusae while others produce fixed gonophores. Moreover, there is simply a lack of knowledge concerning the biology of these cnidarians, and a shortage of people working on them. This shortage is particularly acute at present in North America. In addition to classification and nomenclatural problems in the Hydrozoa, basic identification of taxa in the group is inherently difficult. Hydrozoan taxonomy is confounded by the limited number of useful morphological characters in these anatomically simple animals, together with the wide variations in form exhibited by many species . Moreover, some hydroids can be identified only if 381 19 Hydroids (Cnidaria: Hydrozoa) of the Gulf of Mexico Dale R. Calder and Stephen D. Cairns  Hydroida. After Pratt 1916. 382 ~ Hydroids (Cnidaria: Hydrozoa) Although the classification and nomenclature in Fraser’s synopsis are out of date, the work remains indispensable in the identification of hydroids of the region. When preliminary identifications are made using this reference, the taxonomy of neritic species (occurring at depths of 0–200 m) can be updated by referring to the hydroid sections in Cairns et al. (2002). Other important taxonomic accounts of species occurring in the Gulf include works on hydroids of the Caribbean region by Vervoort (1968), of the Galveston Bay area, Texas, by Defenbaugh and Hopkins (1973), and of Bermuda by Calder (1988, 1991, 1997). An overview of the Stylasteridae of the region is given in Cairns (1986). A summary of literature on hydroids of the Gulf of Mexico through the mid-20th century was provided by Deevey (1954) and will not be repeated here. Much of this information was also incorporated in the guidebook of Fraser (1944). Deevey also included data on species he had identified from the region as part of fouling studies at the Woods Hole Oceanographic Institution during World War II. Nevertheless, he concluded that hydroids of the Gulf of Mexico were little known, and he described his inventory as “a catalog of ignorance.” Since then, a number of works have added to knowledge of the group in the area. A summary of research on hydroids of the northern Gulf, and a table of species reported from that region, were provided by Defenbaugh and Hopkins (1973). Again, that work should be consulted for details. Among the more comprehensive reports since Deevey’s (1950, 1954) publications on Gulf hydroids are (1) a checklist of the biota of Apalachee Bay, Florida (Menzel 1956); (2) a paper on hydroids of Mississippi Sound (Fincher 1955); (3) theses on hydroids of the Florida Gulf coast by Joyce (1961) and Shier (1965); (4) works on fouling organisms, including hydroids, by Gaille (1967) and Pequegnat and Pequegnat (1968); and (5) a checklist of some 184 nominal species reported from the region by Vervoort (1968). Notable works subsequent to the review of Defenbaugh and Hopkins (1973) include (1) a report on the fauna of the West Flower Garden Bank, Texas (Defenbaugh 1974); (2) a monographic thesis on aglaopheniids of the...

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